The invention is in the field of semiconductor devices, and relates more particularly to high-frequency integrated transistor modules.
Bipolar transistors, typically having multiple emitter fingers, can be used in power amplifier circuits suitable for operation at high frequencies in the gigahertz range. However, amplifier circuits employing conventional transistors of this type suffer from a number of drawbacks.
First, when such transistors are used in high-frequency amplifiers, the ground inductance of the circuit limits the gain of the power amplifier. Even a relatively small ground inductance can have a substantial impedance at these high operating frequencies, and costly and space-consuming measures (such as the use of multiple downbonds) must be employed to minimize ground inductance and associated circuit losses.
Second, emitter ballasting in the form of series-connected resistors is typically required for bipolar transistors to avoid thermal runaway and to provide uniform drive when transistors having multiple emitter fingers are used in high-frequency amplifiers. Although such resistors are only required for DC operation, both the high-frequency and DC signals pass through the ballast resistors connected in series with the emitter fingers, thus providing an unwanted attenuation of the high-frequency signal.
Accordingly, it would be desirable to have a high-frequency integrated transistor module suitable for use in high-frequency amplifier circuits in which the drawbacks associated with using the same emitter and ballast resistor connections for both DC and high-frequency signals are minimized.
It is therefore an object of the invention to provide a high-frequency transistor module in which a bipolar transistor having at least one emitter finger with multiple connections is provided, and in which gain reduction due to ground inductance and ballast resistor attenuation is minimized.
It is a further object of the invention to provide a high-frequency transistor module that is compact and economical to manufacture.
In accordance with the invention, these objects are achieved by a new high-frequency transistor module which includes a bipolar transistor having at least one emitter finger, with the emitter finger being internally connected in series with a resistor within the module to provide a DC current path for the transistor module. Additionally, the emitter finger is internally connected in series with a capacitor within the module to provide an RF current path for the transistor module separate from the DC current path.
In a preferred embodiment of the invention, the series-connected capacitor is coupled to an RF ground connection, and the capacitance of the capacitor and the inductance of the RF ground connection are selected to provide gain enhancement in a desired operating frequency range.
In a further preferred embodiment of the invention, the bipolar transistor includes a plurality of emitter fingers, with each emitter finger being internally connected to a respective resistor and a respective capacitor within the module.
High-frequency transistor modules in accordance with the present invention offer a significant improvement in that the effect of signal losses due to ground inductance and emitter ballasting resistors is minimized. Additionally, high-frequency transistor modules in accordance with the present invention are compact in size and economical to manufacture.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments to be described hereinafter.